The present invention is directed to a control system for a bioreactor and more specifically to a method and apparatus for controlling the pressures in a bioreactor to prevent displacement of the livid medium in the cell growth space of a bioreactor.
The culturing of living cells in vitro is performed for a variety of purposes including the preparation of viral vaccines, the recovery of valuable hi-products of cell metabolism and the production of tissue-like derivatives for creating artificial organs.
One method of growing living cells in vitro, which is well known in the art, is hollow fiber technology. Hollow fiber technology provides advantages such as high product concentration, purity and high cell density.
As with all in vitro bioreactors, oxygenation of the cells is a major obstacle to effective and efficient utilization of the cell space.
Traditional hollow fiber methods of providing oxygen to the cells have been to oxygenate liquid medium prior to its delivery to the intercapillary side of the hollow fiber. Oxygen then transfers via diffusion across the fiber wall to the cell environment. This method of delivery is hampered by the fact that aqueous medium, equilibrated with air, can only carry 0.2 mMol of oxygen per liter (37.degree. C., 760 mm Hg). Thus, a large volume of medium must be delivered to the bioreactor to provide a relatively small amount of oxygen.
In accordance with the increasing oxygen requirements of a growing culture, the delivery rates of the oxygenated medium increase. This leads to associated mechanical and biological difficulties as pumps, tubing, fittings in addition to the bioreactor and the medium itself, become subjected to increased pressure and velocity.
A hollow fiber type bioreactor which addresses these problems is constructed in such a way as to allow oxygen to be delivered to the cell environment independently of the medium flow rate.
A hollow fiber-type device which provides for separate oxygen and nutrient media delivery to the cells is disclosed in U.S. Pat. No. 5,079,168 granted Jan. 7, 1992 entitled CELL CULTURE APPARATUS and assigned to Endotronics, Inc. which is a co-Assignee of the present application. This cell culture device is comprised of at least one envelope comprised of first and second membrane layers which are porous and substantially permeable to gases but substantially impermeable to cells and liquids. The envelope is spirally wrapped about an elongated core to define a spirally extending inter-envelope gas space therebetween. Suitable means are provided for delivering nutrient media and cells to the cell culturing space and additional means are provided for delivering a gas to the cell culturing space through the first and second membrane layers. Suitable removal means are also provided for removing liquid metabolic waste products and cell products from the cell culturing space and for removing gaseous waste products from the cell culturing space through the first and second membrane layers.
Another hollow fiber-type device which provides for separate oxygen and nutrient media delivery to cells is also disclosed in co-pending U.S. patent application Ser. No. 07/717,600 filed Jun. 19, 1991, now abandoned entitled CELL CULTURE APPARATUS, which is also assigned to Endotronics, Inc. In this hollow fiber-type device first and second membrane sheets are sealed to each other along their edges to define an envelope with a cell culturing space therebetween. The first and second membrane sheets are porous and substantially permeable to gases but substantially impermeable to cells and liquids. The envelope is disposed between upper and lower rigid plates which form a housing to retain the envelope. A plurality of such envelopes may be stacked within the housing with the housing defining an inter-envelope gas space for the delivery of gas to the cell culturing space through the porous membranes. Suitable delivery means are provided for delivering nutrient media and cells to the cell culturing space and for delivering gas to the inter-envelope gas space. Suitable removal means are also provided for removing metabolic waste products and cell products from the cell culturing space and for removing gaseous waste products from the inter-envelope space.
While the use of gas permeable membranes in direct contact with the cell environment is a highly efficient method of oxygenating cells, the supporting cultureware must be tailored to address problems inherent to this type of bioreactor.
Specifically, the pressure of the cell environment side of the gas permeable membrane must exceed the pressure on the gas side of the gas permeable membrane. If this condition is not maintained, gas will drive across the gas permeable membrane, displacing the cell environments liquid medium and eventually killing the culture. The present invention does not allow this condition to occur.
Additional advantages accrue from utilizing a bioreactor having gas permeable membranes in direct contact with the cell environment as opposed to oxygenating the liquid media being supplied to the bioreactor. Since there is no need to oxygenate the liquid medium entering the bioreactor, flow rates are commensurate with the nutrient delivery needs and waste removal requirements. The systems described in the present application supplies liquid medium directly to the bioreactor simplifying the cultureware complexity and reducing cost.
A problem that results from the elimination of the medium storage vessels is related to the decreased gas solubility of the medium as the temperature of the medium rises. More specifically, the medium is often refrigerated to maintain its life. As it enters the incubator, it increases in temperature and outgasses creating bubbles in the medium. Normally, these would be vented in an intermediate storage vessel, but the direct feed method, according to the present invention, does not permit venting of the bubbles. Thus, the bubbles could form an air lock in the medium delivery path or portions thereof within the bioreactor. The present invention does not allow this condition to occur.